1. Field of the Invention
The present invention relates to the adjustment of a control condition for a reverse rotation process in an injection molding machine having a check valve at the front end of a screw.
2. Description of Related Art
Injection molding machines, such as in-line screw injection molding machines, having an injection mechanism including a check valve mechanism provided at the front end of a screw for preventing resin from flowing backward during injection are already in use. FIG. 1 shows an example of the check valve mechanism. A screw 1 is inserted in a cylinder 7. At the front end of the screw 1, specifically at a portion reduced in diameter between a screw head 2 and a body of the screw 1, a check valve 3 is provided movably in the direction of a screw axis, and at the reduced-diameter portion, on the side near the body of the screw 1, a check seat 4 is provided. The check valve 3 comes in close contact with the check seat 4, thereby blocking a resin passage.
Resin pellets 8 fed from the back of the screw 1 are melted by shearing heat produced by rotation of the screw 1 and heat produced by a heater provided outside the cylinder 7 in which the screw 1 is inserted. The melted resin causes an increase in resin pressure behind the check valve 3, thereby producing a force pushing the check valve 3 forward. When the check valve 3 is pushed forward, the resin behind flows forward of the check valve 3, through a gap between the check valve 3 and the reduced-diameter portion, thereby increasing the pressure in front of the screw head 2 within the cylinder 7.
When the pressure in front of the check valve 3 exceeds a specified value (back pressure), the screw 1 is pushed backward, so that the pressure in front of the check valve 3 decreases. Further rotation of the screw 1 increases the pressure behind the check valve 3 to become higher than the pressure in front of the check valve 3. Consequently, the melted resin is fed forward of the check valve 3, continuously. When the screw 1 moves backward up to a specified position, the rotation of the screw 1 is stopped (metering process).
Then, in an injection process, when the screw 1 is moved forward (from the right to the left in FIG. 1) to inject resin, the pressure of the resin stored in front of the screw head 2 increases, so that the check valve 3 moves backward and comes in close contact with the check seat 4, thereby blocking the resin passage to prevent the melted resin from flowing backward (as indicated by an arrow F), i.e., in the direction of backward motion of the screw, due to injection pressure. Variation in timing at which the check valve 3 moving backward blocks the resin passage results in variation in the amount of resin injected, which leads to unstable molding.
In the injection process, the check valve mechanism is closed by the pressure in front of the check valve mechanism which is increased by the forward motion of the screw 1 to become higher than the pressure behind. Immediately before injection, however, there is residual pressure behind the check valve mechanism, in grooves 6 between flights 5, as mentioned above, and such residual pressure causes variation in timing of blocking. After the start of injection until the check valve blocks the resin passage, resin flows backward from in front of the check valve to behind occurs. Thus, the variation in blocking timing results in variation in volume injected per cycle, which affects the quality of products molded.
In this connection, there is known an invention in which a step of rotating the screw in the opposite direction to the direction that the screw is rotated in the metering process is added after the completion of the metering process and before the injection/pressure holding process, so that injection is performed after the screw is rotated reversely (see JP45-2988B, JP56-113440A, and JP01-192521A). Although the reverse rotation of the screw does not always ensure that the check valve is closed at the start of injection, the screw is rotated reversely to ensure that the check valve is closed immediately after injection starts.
For such reverse rotation of the screw, a control condition for reverse rotation, such as reverse rotation amount, reverse rotation speed or the like, needs to be set to an appropriate value.
In connection with a method of determining the control condition for reverse rotation, for example JP04-284221A discloses an invention in which, utilizing the phenomenon that the closure of the check valve during the screw reverse rotation causes a steep increase in pressure of oil supplied to an oil motor for rotating the screw, the reverse rotation of the screw is stopped when torque reaches a specified value during the screw reverse rotation.
JP2003-305758A discloses an invention in which a plurality of sets of values of control conditions for screw reverse rotation (screw reverse rotation time, pressure applied in the direction of screw forward motion, reverse rotation speed and reverse rotation amount) are prepared depending on resin viscosity to allow automatic setting of the control conditions for reverse rotation by selecting one set of values.
If injection is performed with the check valve opened, resin flows backward during the injection, so that torque in the opposite direction to the direction of screw rotation in the metering process (reverse torque) is exerted on the screw. There are known an invention in which the reverse rotation of the screw caused by such reverse torque is prevented by supplying a small amount of hydraulic pressure to a hydraulic motor for rotating the screw (see JP48-21741A and JP49-74252A), an invention in which such rotation of the screw is prevented by feeding a rotational speed instruction 0 to a motor for rotating the screw or performing position control over the motor (see JP62-236720A), and an invention in which such rotation is prevented using a ratchet (see JP63-166412U).
Further, regarding the injection molding machine with the check valve at the front end of the screw, it is known that when the check valve becomes closed during injection, reverse torque ceases to be exerted on the screw.
For example, JP11-170319A discloses an invention in which, at the start of injection, the reverse rotation of the screw is prevented by applying a braking force, and when the screw moves forward a specified distance, it is determined that the check valve is closed and the force braking the screw rotation is removed, since the closure of the check valve stops reverse torque being exerted on the screw.
Further, JP2004-216808A discloses an invention in which injection is started allowing the screw to rotate in either direction, so that at the start of injection, the screw rotates reversely due to the back flow of resin. In the present invention, the closure of the check valve is detected by detecting the stop of the screw reverse rotation, since the closure of the check valve stops reverse torque being exerted on the screw.
Further, JP01-168421A disclose a technique of detecting an abnormality by detecting a reverse torque exceeding an allowable range, which technique is based on the fact that an increase in resin back flow in the injection/pressure holding process due to wear of the check valve, etc. results in an increase in reverse torque on the screw in the injection/pressure holding process.
Variation in back-flow amount due to resin back flow produced during injection results in variation in the amount of resin injected into a mold and affects the quality of products molded. Thus, it is necessary to rotate the screw reversely, thereby producing a pressure difference between in front of and behind the check valve, thereby causing the check valve to be moved and closed or to be more likely to be closed. For this, it is desired to obtain a more suitable value of the control condition for screw reverse rotation.
Insufficient reverse rotation amount in this screw reverse rotation process results in an increase in back-flow amount in injection and affects the quality of products molded. Conversely, excessive reverse rotation amount causes the resin behind the check valve to be sent backward more than required, which results in an increase in metering time, and therefore an increase in cycle time.
As mentioned above, in the invention disclosed in JP04-284221A, utilizing the phenomenon that the closure of the check valve during the screw reverse rotation causes a steep increase in pressure of oil supplied to the oil motor for rotating the screw, the reverse rotation of the screw is stopped when torque reaches a specified value during the screw reverse rotation. There are, however, cases that, due to the type of resin, the reverse rotation of the screw is not enough to cause the closure of the check valve, or the closure of the ring during the screw reverse rotation does not cause an increase in reverse torque. Thus, the invention disclosed in JP04-284221A has a problem that an optimal value of the control condition for screw reverse rotation is difficult to obtain.
In the invention disclosed in JP2003-305785A, values of control conditions for screw reverse rotation are determined depending only on resin viscosity. The optimal value of reverse rotation amount, however, varies depending on the shape of the mold, the shape of the screw, the stroke of the check valve, and other conditions. Thus, the method disclosed in JP2003-305758A has a problem that the reverse rotation amount is not always adjusted to an optimal value.